1. Field of the Invention
The invention relates to a process for preparing discolouration-stable polyisocyanate prepolymers containing allophanate structural units by using specific polyethers, to the process products thus obtainable and to their use for preparing polyurethanes and polyureas.
2. Description of the Prior Art
Polyisocyanate prepolymers containing allophanate structural units are of particular interest on account of their high NCO content at comparatively low viscosity. They represent useful crosslinkers for two-component polyurethane systems and with blocked NCO groups they can also be used in one-component polyurethane systems. Polyurethane systems of this kind are generally employed for producing coatings.
Polyisocyanate prepolymers containing allophanate structural units are known in principle. Thus EP-A 0 682 012, among other things, describes prepolymers based on diisocyanates and polyethers containing 1–4 hydroxyl groups, which are reacted using tin(II) compounds with an excess of the diisocyanates to give the corresponding allophanates.
According to the processes described, however, the polyisocyanate prepolymers obtained are frequently products which on storage, particularly at elevated temperatures, exhibit a distinct colour change. Furthermore, in the course of the said storage, there is often a decrease in the NCO content of the products and a considerable increase in their viscosity. Since the polyisocyanate prepolymers containing allophanate structural units are generally freed from excess diisocyanate by means of thin-film evaporation at high temperatures (e.g. 160° C.), the changes referred to (in colour, NCO content and viscosity) frequently occur even during preparation.
It was therefore an object of the present invention to provide new polyisocyanate prepolymers containing allophanate structural units which are endowed with markedly improved storage stability, and in particular with improved colour stability.